It is well known that solid particulates and larger particles may be filtered from fluids (i.e., gases and/or liquids) by passing the particulate contaminated fluids through porous, walled honeycomb structures. U.S. Pat. No. 4,329,162 describes and claims honeycomb filters for removing carbonaceous solid particulates from diesel engine exhausts and other filtering applications. A typical diesel particulate filter ("DPF") has a multiplicity of interconnected thin porous walls which define at least one inlet surface and one outlet surface on the filter and a multiplicity of hollow passages, or cells, extending through the filter from an inlet surface or an outlet surface or both. Inlet cells are open at least one inlet surface to admit contaminated fluid into the filter. The inlet cells are closed where they adjoin any outlet surface of the filter. Outlet cells are formed open at an outlet surface to discharge fluid which has passed through the filter. The outlet cells are similarly closed where they adjoin any inlet surface. The interconnected thin walls are provided with an internal interconnected open porosity which allows the fluid to pass from the inlet to the outlet cells while restraining a desired portion of the solid particulates in the fluid.
The particulates are trapped in or collected on the surfaces of the thin walls defining the inlet cells. As the mass of collected particulates increases, back pressure across the filter increases and/or the flow rate of fluid through the filter decreases until an undesirable level of back pressure and/or flow rate is reached and the filter either is regenerated by removal of the trapped particulates or discarded. DPFs are typically installed in a housing which, like a muffler or catalytic converter, is inserted into the exhaust system of a diesel engine equipped vehicle.
To produce the required porosity in a ceramic substrate to be used as a particulate filter, a "burnout" material is commonly added to and mixed with ceramic precursors prior to firing. This pore-forming material is burned out when the ceramic precursors are fired to produce the hardened ceramic body. The most common burnout material used in ceramic articles is graphite because it produces pores of optimal size and good overall porosity without swelling which can cause cracking or weakening of the ceramic article.
Although ceramic ware prepared with up to 30 weight percent graphite exhibit acceptable physical properties, graphite burnout material is not without disadvantages. The most severe problem is the inability completely to dry graphite-containing ceramic ware dielectrically. A dielectric dryer utilizes a pair of opposing plates or electrodes to create a high frequency electrical field between the plates or electrodes. This "dielectric" field couples with the water in the ware, resulting in absorption of energy by the water. This energy absorption results in heating and evaporation of the water in the ware. Dielectric drying is the preferred method of drying ceramic ware because of the speed and uniformity with which the ceramic articles are dried. In addition, dielectric drying decreases cracking of the article during drying and increases the dimensional accuracy of the finished ware.
It has been found, however, that if the formed ceramic substrates containing a high level of graphite are dried dielectrically beyond some point (and before drying is complete), arcing or shorting takes place between the electrodes of the dryer and the ceramic ware. Arcing in the dielectric dryer can cause many problems including burning of the ware, cracking, or damage to the dryer. Also, because the ware cannot be fully dried dielectrically, drying must generally be completed in a conventional hot air oven. Due to the size of the ware typically used as particulate filters and non-uniformity of drying, considerable cracking of the ceramic can occur during hot air drying. Lastly, the use of graphite to develop porosity in the ceramic article results in a large exothermic reaction when the graphite is burned out. The reaction causes the inside of the ware to get much hotter than the outside during firing. These severe thermal gradients are another cause of cracking.
Therefore, although the use of graphite as a burnout material has resulted in ceramic wares exhibiting good physical properties, there continues to be a need to improve the method of producing dimensionally accurate, durable porous ceramic substrates.